LABRADORITE

Description:

Labradorite is a fascinatingly beautiful feldspar mineral. Labradorite can produce a colorful play of light across cleavage planes and in sliced sections called labradorescence. The usually intense colors range from the typical blues and violets through greens, yellows and oranges. Some rare specimens display all these colors simultaneously.

The color display is from lamellar intergrowths inside the crystal. These intergrowths result from compatible chemistries at high temperatures becoming incompatible at lower temperatures and thus a separating and layering of these two phases. The resulting color effect is caused by a ray of light entering a layer and being refracted back and forth by deeper layers. The effect depends on the thickness and orientation of the layers.

Labradorite is a member of the plagioclase series of minerals, and must contain 50-70% calcium to 50-30% sodium in the sodium/calcium position of the crystal structure. All members of the plagioclase series usually display lamellar twinning called "Albite Twinning". The twinning is caused by an error in the crystal structure during its growth, and produces a layered or stacked effect. The twin layers are typically only fractions of millimeters to several millimeters thick.

Labradorite occurs as clear, white to gray blocky to lath shaped grains in common mafic igneous rocks of low silica content. In sliced sections of rock, the labradorite appears as blocky chunks with a predominance of near right angled corners. The geological type area for labradorite is Paul's Island in Labrador, Canada. Other notable occurrences include Scandinavian Peninsula.

Labradorite has become a popular semiprecious stone—fairly durable, attractive in large pieces, and inexpensive.

Characteristics:

Chemistry: NaAlSi₃O₈ – CaAlSi₂O₈, Calcium sodium aluminum silicate

Class: silicates

Hardness (Mohs scale): 6 – 6.5

Density: 2.68 – 2.71

Refractive index: 1.555 – 1.572

Dispersion: 0.012

Crystal system: triclinic

Crystal habit: well-formed coarse sized crystals that rarely form free of the host rock

Fracture: uneven

Cleavage: in three directions, two of which form nearly right angled prisms

Transparency: translucent to transparent

Luster: vitreous